A metal-air cell is a type of battery that utilizes the same energy storage principles as a more conventional cell such as a lithium ion, nickel metal hydride, nickel cadmium, or other cell type. Unlike such conventional cells, however, a metal-air cell utilizes oxygen as one of the electrodes, typically passing the oxygen through a porous metal electrode. The exact nature of the reaction that occurs in a metal-air battery depends upon the metal used in the anode and the composition of the electrolyte. Exemplary metals used in the construction of the anode include zinc, aluminum, magnesium, iron, lithium and vanadium. The cathode in such cells is typically fabricated from a porous structure with the necessary catalytic properties for the oxygen reaction. A suitable electrolyte, such as potassium hydroxide in the case of a zinc-air battery, provides the necessary ionic conductivity between the electrodes while a separator prevents short circuits between the battery electrodes.
Due to the use of oxygen as one of the reactants, metal-air cells have some rather unique properties. For example, since the oxygen does not need to be packaged within the cell, a metal-air cell typically exhibits a much higher capacity-to-volume, or capacity-to-weight, ratio than other cell types making them an ideal candidate for weight sensitive applications or those requiring high energy densities.
Regardless of the composition and mechanical nature of the elements used in a metal-air battery, oxygen is required for the reaction to take place. Therefore during the discharge cycle, the reaction rate of the cell may be varied by controlling the flow of oxygen into the cell. During the charging cycle, the metal oxides or ions are reduced to form the metal comprising the anode and oxygen is emitted by the cell.
While metal-air cells offer a number of advantages over a conventional rechargeable battery, most notably their extremely high energy density, such cells also have a number of drawbacks. For example, care must be taken to insure a sufficient supply of air to the cells during discharge cycles, and means for handling the oxygen emitted from the cells during the charge cycles, both of these issues becoming increasingly important as the number of metal-air cells and/or the size of the cells increase to meet the demands of larger applications.
Accordingly, while metal-air cells offer some intriguing benefits, such as their high energy densities, their shortcomings must be taken into account in order to successfully integrate the cells into a system.